Microbes change the chemistry of deep ocean vents

Most of the focus of hydrothermal vents has been on the high-temperature, …

Hydrothermal vents on the sea floor have been full of surprises, starting with their unexpected discovery in the late 1970s. Entire communities thrive in the complete absence of sunlight, extracting energy from the unique chemistry of the vents. We’ve also learned that hydrothermal vents play an important role in marine chemical cycles. At times in the past, changes in hydrothermal circulation have even switched the dominant form of carbonate precipitate in the ocean from calcite to a form higher in magnesium called aragonite.

Most of the focus thus far has been on the high-temperature (up to 320°C), focused vents of television documentary fame. But recent research is showing that lower-temperature (10-80°C) sites where discharge is more diffuse are just as interesting, and have a few surprises of their own.

Research at hydrothermal vents has long been limited by the difficulty of obtaining water samples and the constraints of analysis that typically has to be performed in a lab far removed from the sea floor. A team of researchers operating from a submersible has deployed new devices that can measure chemistry and flow rate in situ, and have been able to study the vents along the Juan de Fuca ridge off the coast of Washington in much greater detail.

Using chemical concentrations and flow rates from both types of vents, the researchers were able to calculate a first-order estimate of mass flux. The first surprise they report in their paper, published in Nature Geoscience, is that the lower-temperature releases actually account for at least half of the geochemical contribution to the ocean made by hydrothermal vents. While the concentrations and flow rates are lower, these vents are more common than their high-temperature counterparts.

Second, their data also includes some interesting discrepancies that point to a strong biological influence on the chemistry of the water coming out of the lower-temperature vents. The concentrations of hydrogen gas were 50 percent to 80 percent lower than chemically predicted, indicating that something must be using it up. The researchers chalk this up to hydrogen-oxidizing bacteria and methane-producing bacteria (known to live in these environments), showing that some of the smallest forms life may be greatly altering the chemistry of hydrothermal vents. It seems that low-temperature vents are likely to become hot spots for research.